1. Field of the Invention
This invention relates to a novel method for the preparation of diphenylsiloxane-dimethylsiloxane copolymers. More particularly, this invention relates to a method for preparing diphenylsiloxane/dimethylsiloxane copolymers without generating substantial amounts of cyclic organosiloxanes or undesirably high molecular weight polymers.
2. Background Information
It is known to prepare diphenylsiloxane-dimethylsiloxane copolymers by the polymerization of mixtures containing hexaphenyltrisiloxane and hexamethylcyclotrisiloxane in the presence of lithium compounds, including the hydroxide and silanolates, as polymerization catalysts. The preparation of block copolymers by the sequential addition of these cyclosiloxanes to a reaction mixture containing lithium silanolates or complexes of these silanolates is described in U.S. Pat. Nos. 3,578,726 and 3,337,497, which issued to Bostic.
Polymerization of a mixture of cyclic dimethyl- and diphenylsiloxanes in the proportion required to prepare a copolymer using a silanolate such as potassium trimethylsilanolate is taught in U.S. Pat. No. 4,807,341, which issued to Nielsen.
The disadvantages inherent in these previously described preparative methods include low yields of the desired copolymer due to the production of large amounts of cyclic organosiloxanes when the initially formed polymers are allowed to equilibrate. The quantity of these cyclic organosiloxanes makes it difficult to control the molecular weight of the desired polymer. In this type of polymerization the molecular weight must be controlled empirically by adjusting the quantity of reactant used to form the terminal groups of the polymer molecule.
When water or a low alpha, omega-dihydroxy-terminated oligomeric diorganosiloxane is used as the end-blocking agent to obtain a copolymer terminated by a silanol group at both molecular-chain terminals, these silanol groups can react with one another with the resultant elimination of water and formation of a siloxane (Si--O--Si) bond. Under these conditions the molecular weight of the copolymer will far exceed the desired value.
One objective of the present invention is to provide a method for the preparation of diphenylsiloxane-dimethylsiloxane copolymer which provides the copolymer in high yields and in the desired molecular weight range.